1. Field of the Invention
The present invention relates to a small zoom lens system which is used in a video camera and a digital camera, etc., and has a zoom ratio of about 10:1, and in particular, relates to such a small zoom lens system of a four-lens-group arrangement, i.e., a lens group having a positive refractive power (hereinafter a positive lens group), a lens group having a negative refractive power (hereinafter, a negative lens group), another positive lens group, and the other positive lens group, in this order from the object.
2. Description of the Prior Art
In a zoom lens system of the four-lens-group arrangement, i.e., a positive lens group, a negative lens group, another positive lens group, and the other positive lens group, in this order from the object, a rear-focusing system, in which the fourth (final) lens group (i.e., the most image-side lens group) is arranged to be a focusing lens group, has commonly been employed. However, in such a rear-focusing type zoom lens system, if the shortest photographic distance is made further shorter, the traveling distance of the focusing lens group becomes longer, so that it takes longer time to perform a focusing operation. Moreover, in the rear-focusing type zoom lens system, it has been known that filed curvature largely and undesirably occurs therein.
In addition to the above, an inner-focusing type zoom lens system in which the second lens group is arranged to be a focusing lens group has been known in the art. However, the second lens group is in general used as a zooming lens group, so that the lateral magnification of the second lens group largely varies when the second lens group is being moved for zooming. Accordingly, during the movement of the second lens group, it is often the case where the lateral magnification the second lens group becomes 1.0. If this occurs, focusing cannot be performed thereat.
If an attempt is made to avoid the lateral magnification of 1.0 (life size) during a focusing operation, the zooming range has to be designed with respect to a less-than life size magnification (reducing magnification) only, or with respect to a more-than life size (enlarging magnification) only, respectively. However, such an arrangement limits the degree of freedom on optical design.
Furthermore, in order to miniaturize the zoom lens system, the lens arrangements of the third and fourth lens groups which are arranged to form an image by collecting a diverging light rays emitted from the second lens group becomes increasingly important.